If you're going by the religious reasoning, it's actually Saturday which was the 7th day.
That part comes from the Old Testament which comes from the Judaic scriptures and for them, the 7th day that has been observed for thousands of years is what in modern time is a Saturday - well it starts at 6pm on the Friday and goes through to 6pm on the Saturday - the Sabbath.
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My body is an old warehouse full of declining storage, my mind is a dusty old reference library, strictly for members and archaeologists only
